T3 FAILS TO MIMIC CERTAIN EFFECTS OF T4 IN MUNIA BIRDS - PHYSIOLOGICAL IMPLICATIONS FOR SEASONAL TIMING

The comparative effects of equimolar doses of thyroxine (T4) and tri-i odothyronine (T3) were studied on fattening, gonadal status and moulti ng in thyroidectomized female spotted munia, The effects of suppressio n of peripheral conversion of T4 to T3 were also studied on these para meters, T4 administration led within 10 days to a significant regressi on of gonads and mobilization of fat, the response being linear, T3 ha d no effect on these parameters at any dose level, However, both T4 an d T3 were effective, the former to a greater extent in stimulating fea ther regeneration in the same birds. Inhibition of peripheral monodeio dination of exogenous T4 in thyroidectomized (TX) birds by iopanoic ac id (IOP) did not suppress T4 response, but resulted in greater regress ion of gonads and fat mobilization; however, IOP alone was ineffective , Long-term suppression of peripheral monodeiodination of endogenous T 4 in intact birds with IOP suppressed follicular growth and fat deposi tion but enhanced moult, Hormonal profiles of T4 and T3 indicated that seasonal moult-inducing, anti-gonadal and anti-fattening effects of t he thyroid gland are reflected by circulating T4 but not T3 in the mun ia bird, It may be concluded that T4 may be more potent than T3 in eli citing certain biological actions and that T3 may not mimic all T4 eff ects, Contrary to general opinion T4-induced effects need not necessar ily be mediated through its prior conversion to T3, giving it an indep endent hormonal role, This may be a mechanism evolved by passerine bir ds which are generally known to be iodine-deficient, to preclude the s imultaneous occurrence of seasonal events requiring high energy and nu trition, such as moulting, reproduction and migration.